Method for trimming glass

ABSTRACT

Method for improving the edge quality of bulb edge trim cuts wherein a band of the glass parallel to a bulb edge is heated during the cutting operation so as to reduce compressive forces in the bulb portion.

' United States Patent Dahlberg et a1.

[ 1 Nov. 25, 1975 METHOD FOR TRIMMING GLASS lnventors: John R. Dahlberg, Jeannette: Robert H. Grove, Sarvar, both of Pa.

Assignee: PPG Industries, Inc., Pittsburgh, Pa.

Filed: Dec. 7, 1973 Appl. No: 422,949

Published under the Trial Voluntary Protest Program on January 28, 1975 as document no. B 422,949.

US. Cl. 225/2; 225/935; 225/965 Int. Cl. B26F 3/00 Field of Search 225/2, 93.5, 94, 96, 96.5

References Cited UNITED STATES PATENTS Wright 225/915 2,310,469 2/1943 Snyder 225/93.5 3,344,968 10/1967 Kovacik 225/935 X 3,754,884 8/1973 McDavid et al. 225/9615 X 3,800,991 4/1974 Grove et al. 225/965 X Primary Examiner-Frank T. Yost Assislant Examiner-Leon Gilden Attorney, Agent, or Firm-Dennis G. Millman [57] ABSTRACT Method for improving the edge quality of bulb edge trim cuts wherein a band of the glass parallel to a bulb edge is heated during the cutting operation so as to reduce compressive forces in the bulb portion.

13 Claims, 3 Drawing Figures U.S. Patent Nov. 25, 1975 Sheet10f2 3,921,873

US. Patent Nov. 25, 1975 Sheet20f2 3,921,873

METHOD FOR TRIMMING GLASS BACKGROUND OF THE INVENTION This invention relates to a method of severing glass, and more particularly to an improvement in the method of trimming the so-called bulb edge from sheets of glass.

When a ribbon of glass exits from the sheet-forming process, its longitudinal edges are found to have assumed an irregular, rounded shape known as the bulb edge. Since the vast majority of the commercial uses of glass require a straight, square edge, it is necessary to trim the bulb edge from the body of the piece. Removal of the bulb edge is also helpful in reducing breakage when sheets of thinner glass are stacked for storage. Trimming the bulb edge using prior art techniques, however, usually does not result in a commercially acceptable edge due to the formation of shark teeth" (visibly prominent propagation markings on the face of the open cut) and hackle" (rough edges) as well as bevel (deviation of the cut edge from perpendicular to the major surfaces). These effects become more severe with thicker glass, and are especially prominent on float glass. The presence of these edge defects has required making a second trim cut, and sometimes a third and even a fourth cut, in order to achieve an acceptable edge. Such an approach is shown in US. Pat. No. 3,344,968 to Kovacik et al. The prior art approach not only has the disadvantage of requiring at least two cutting operations, but also results in an additional trimmed portion of the sheet being wasted.

Another disadvantage with conventional bulb edge trimming is that cuts cannot be run continuously on longer pieces of glass or ribbons, but must be snapped section by section after cross-cutting in order to confine the cut to the intended line. Also, since severe warping of glass sheets is often encountered before the bulb edge is removed, especially with thinner glass, the conventional use of mechanical cutters to remove the bulb edge can result in high breakage rates.

SUMMARY OF THE INVENTION The present invention provides for a method of trimming the bulb edge from a sheet of glass so as to obtain a commercially acceptable edge with a single trim cut. The invention also permits continuous cut-running of bulb edge trim cuts on long sheets or ribbons.

The poor quality of the usual first trim cut along a bulb edge has been found to be attributable to the presence in the bulb edge portion of compressional forces having large magnitudes relative to the forces found in the body of the sheet. This uneven force pattern is frozen" into the glass during the formation process due to the edge portions cooling at a different rate than does the body of the sheet. Although not fully understood. it is theorized that these pent-up forces cause a shearing effect during propagation of a cut along the bulb edge, which results in the observed poor quality edges. Although the effects are especially severe when cutting near the bulb edge in the compression zone itself, the forces in the bulb appear to affect cuts located far into the body of a sheet also.

We have found that heating a parallel band of the glass sheet on the body side of the intended path of cut can create a counteracting compressional force that will sufficiently relax the compression in the bulb edge to permit normal cut propagation of bulb edge trim cuts. The heat-induced compression in the band causes a slight expansion of a portion of the body of the sheet, which in turn causes the bulb edge to expand, thereby relieving the compressional forces in the bulb portion. It has been found that if a bulb edge trim cut is made while the bulb edge portion remains in such a relaxed state. the resultant cut is square and free from defects. so that no second trim cut is necessary. Ideally the compression along the path of cut would be reduced to zero, but any degree of compression reduction will yield a corresponding improvement.

DETAILED DESCRIPTION OF THE INVENTION The invention will be better understood from the following detailed description taken together with the drawings, in which:

FIG. 1 is a perspective view of a bulb-edged piece of glass undergoing trimming in accordance with one embodiment of the present invention;

FIG. 2 is a perspective view of another embodiment of the invention adapted to remove bulb edges on two sides of a piece of glass; and

FIG. 3 is a perspective view of yet another embodiment of the invention adapted for use on a continuous production line. i

In FIG. 1, there is shown a sheet of glass 10 having a bulb edge 12 supported on table 14. When it is desired to trim off the bulb edge along a line 15, a band 16 of the sheet is heated with a heater 20. A scoring tool 22 is then passed along the intended path of cut to produce score line 15 and the glass severed by applying a bending moment while band 16 remains hot. The piece of glass may remain stationary while the heater 20 and scorer 22 traverse the length of the piece as shown in FIG. 1, or the glass may be conveyed past a stationary heater and cutter.

Heater 20 may be any suitable source of heat. such as a radiant infrared lamp as shown in FIG. 1, an open flame burner (e.g., an array of gas torches), or convective air jet heaters such as those disclosed in US. Pat. No. 3,730,408. The heating capability required depends upon a number of factors such as the thickness of the glass, speed required, width of the heated band and the magnitude of the compression. It has been found advantageous to produce the counteracting compressive stresses with a slight temperature increase in a relatively wide band, thus permitting the use of safer, lower temperatures and faster heating times; Temperatures so high as to cause fracturing in the band or melting are of course avoided. Heating the band to a temperature of only about 30F. or less above the glass temperature can produce the required thermal expansion in a heated band several inches wide. In this temperature range, band widths on the order of two inches to six inches have been found to produce significant relaxation of the bulb on 541 inch float glass, for example, but the width may be varied considerably as a function of glass thickness. In the embodiment shown in FIG. 2, a single wide band 24, which may encompass essentially the entire center portion of a sheet 26, may be heated by means of a longer heat source 29, such as the infrared heater shown in FIG. 2, so as to permit removal of both bulb edges 21 and 23 simultaneously along sc'ore lines 27 and 28 respectively.

Although the temperatures noted herein are all taken at the surface of the glass, it is observed that relaxation of the bulb is more strongly affected if the entire thickness of the glass becomes heated. Thus it is preferred to 3 provide a few seconds delay for the heat to "soak in" and/or to heat the glass from both sides of the sheet.

An important aspect of the invention is the location of the band that is heated. Not only must the heat band be spaced from the bulb edge so as to not aggrevate the compressional forces therein, but it must be spaced from the intended line of cut as well. The presence of heat in the glass near the intended line of cut can cause the propagation of the cut to be drawn away from the intended line and toward the heat. Thus the space between the heated band and the line of cut must be wide enough to insulate against heat conduction long enough for the severing to be completed. The precise spacing required will vary from case to case, but about two inches is considered the minimum. On the other hand, the maximum width is limited only by the practicality of transmitting the forces generated in the heated band to the bulb edge.

The quality of the cut edge is also affected by the location of the line of cut relative to the edge of the piece of glass. Although improved edge quality can be obtained when practicing the present invention regardless of the location of the line of cut, the best results are obtainable when cutting a few inches away from the bulb edge. Good edges are achieved, for example, when the cut is spaced two inches from the edge on inch glass, three inches on /s inch glass, and 4 inches on /2 inch glass.

The actual cutting operation may utilize any known glass cutting technique. One such technique is the conventional use of scoring wheels 22 as shown in FIGS. 1 and 2 to score the surface of the glass, followed by application of a bending moment about the score line to complete the fracture. Also suitable are the thermal cutting techniques disclosed in U.S. Pat. Nos.

3,695,497; 3,695,498; and 3,730,408, wherein thermal energy alone or in combination with a mechanical bending moment may be utilized. In FIG. 3, there is shown an arrangement incorporating a thermal cutting means 34 of the type disclosed in U.S. Pat. No. 3,695,497, i.e., an infrared spot heater. A fractured trim section 36 can be subsequently removed by making a small score such as 37 and tapping with a mallet 38.

Since the squareness of a face of a cut edge can be affected by the presence of heat, care must be taken to avoid subsequently cross-cutting the glass perpendicularly to the trimmed edge while the heated band remains hot. Either a cooling-down period must be allowed, or the cross-cutting must be done upstream from the bulb edge trimming operation to avoid erratic cross cuts.

EXAMPLE It was desired to remove the bulb edge from a 48 inch X 128 inch X inch piece of float glass with the bulb on one 128 inch edge. A 6 inch wide rectangular band running the full length of the piece was heated using a 6 inch long radiant infrared Line Heater (characterized by a tungsten filament in a gas-filled quartz envelope, having an elliptical reflector, and manufactured by Research, Inc., Minneapolis, Minn.) leaving an 8 inch wide unheated space between the band and the edge of the glass having the bulb. The heater was translatably mounted above and closely adjacent to the surface of the glass perpendicular to the bulb edge in an arrangement similar to that shown in FIG. 1, and was operated by uniformly scanning the surface of the glass repeatedly until the temperature on the surface of the resulting heated band was about 30F. above the initial glass temperature. A score was then made along a line 2 inches from the bulb edge, and the full length of the bulb edge trim severed with cut running pliers. The quality of the resulting cut edge was considered excellent, and no further trimming was needed. By comparison, without the application of the heated band, the same cut yielded an edge that had an unacceptable amount of defect and the cut ran only about a fourth of the length before deviating from the intended line of cut. In the above Example, the compression along the intended line of cut was measured to have been reduced by the heating of the band to amounts ranging from zero to less than a fourth of the normal amount. In cases where it may be desired to obtain a uniform resulting compression along the path of cut, this can be achieved by shaping the heated band or by modulating the heater output as it scans the band.

The arrangement set forth in the Example may be readily adapted to continuous production line use as shown in FIG. 3 by replacing the infrared heater with a 6 inch long row of propane torches 30 extending across the width of the heated band 32 on continuous ribbon 35 and adjusting the flames so as to yield the required temperature increase with a single passage of the glass. It should be understood that conventional mechanical scoring and breaking means may be utilized in place of the thermal means 34 shown in FIG. 3.

Although described with particular reference to bulb edge removal from sheets, it should be apparent that the invention is equally applicable to any glass cutting operation where it is desired to sever a portion in compression from a less compressed portion.

We claim:

1. A method of cutting a piece of glass along an intended path of cut substantially parallel to an edge portion which exhibits preformed internal stresses which are compressive relative to the stresses in the main body of the piece comprising:

heating a band of the piece parallel to said edge portion and spaced from the intended path of cut in the direction away from said edge portion so as to effect a relaxation of the stresses in said edge portion; and

severing the piece along said intended path of cut before the heating effects dissipate from said band.

2. The method of claim 1 wherein the temperature of the heated band is limited to temperatures below those at which the glass would fracture or melt.

3. The method of claim 2 wherein the temperature at the surface of the heated band during the severing step, is about 30F. above its initial temperature.

4. The method of claim 3 wherein said heated band is at least about two inches wide.

5. The method of claim 1 wherein the space between the heated band and the intended path of cut is of sufficient width to insulate the intended path of cut from the heat in said band during the severing operation.

6. The method of claim 5 wherein said space is at least about two inches wide.

7, The method of claim 1 wherein the severing step comprises:

mechanically scoring the surface of the glass along the intended path of cut; and

applying a bending moment about the intended path of cut to fracture the glass.

10. The method of claim 1 wherein said piece of glass is continuously conveyed past means to perform said heating of a band and means to carry out said severing.

1]. The method of claim 1 wherein said piece of glass is flat.

12. The method of claim I wherein said piece of glass is a continuous ribbon.

13. The method of claim 1 wherein a cut is made along each of two opposite internally compressed edge portions and a single centrally located band is heated. s 

1. A method of cutting a piece of glass along an intended path of cut substantially parallel to an edge portion which exhibits preformed internal stresses which are compressive relative to the stresses in the main body of the piece comprising: heating a band of the piece parallel to said edge portion and spaced from the intended path of cut in the direction away from said edge portion so as to effect a relaxation of the stresses in said edge portion; and severing the piece along said intended path of cut before the heating effects dissipate from said band.
 2. The method of claim 1 wherein the temperature of the heated band is limited to temperatures below those at which the glass would fracture or melt.
 3. The method of claim 2 wherein the temperature at the surface of the heated band during the severing step, is about 30*F. above its initial temperature.
 4. The method of claim 3 wherein said heated band is at least about two inches wide.
 5. The method of claim 1 wherein the space between the heated band and the intended path of cut is of sufficient width to insulate the intended path of cut from the heat in said band during the severing operation.
 6. The method of claim 5 wherein said space is at least about two inches wide.
 7. The method of claim 1 wherein the severing step comprises: mechanically scoring the surface of the glass along the intended path of cut; and applying a bending moment about the intended path of cut to fracture the glass.
 8. The method of claim 1 wherein the severing step comprises applying thermal energy to the intended path of cut.
 9. The method of claim 1 wherein the severing step comprises applying thermal energy to the intended path of cut and subsequently applying a bending moment about the intended path of cut.
 10. The method of claim 1 wherein said piece of glass is continuously conveyed past means to perform said heating of a band and means to carry out said severing.
 11. The method of claim 1 wherein said piece of glass is flat.
 12. The method of claim 1 wherein said piece of glass is a continuous ribbon.
 13. The method of claim 1 wherein a cut is made along each of two opposite internally compressed edge portions and a single centrally located band is heated. 